We're currently using an iPAQ for wireless work in our warehouse. We purchased a Symbol 3000 wireless LAN/ barcode reader backpack for it.
Anyway, I developed an app using PHP, MySQL backend, and it works great (interfaces with our inventory management system). The iPAQ is fantastic for this application, although it does have it's limitations.
First, your working with Pocket IE. This is the server strings it gives: "Mozilla/2.0 (compatible; MSIE 3.02; Windows CE; PPC; 240x320)". Yup, IE 3.02. So your limited in what you can do. It can be frustrating manipulating tables and getting your data to look right. Text size is very strange. You need to boost the size a bit. Forget using stylesheets, or using a <p style="this:that;"> type tag. No styles of anykind work. You cannot popup small windows, and I'd avoid using target="_blank" in your anchor tags.
You didn't mention what type of wireless device you want. But be warned: ALL versions of IE on these devices are limited to what I mentioned above.
Other than that, the PHP functionality is like that of a desktop, just displayed smaller.
Hope that helps.
a9